Some MOSFETS include vertical trenchFETs having polysilicon filled gate trenches in an active region and a polysilicon filled termination trench which surrounds the active region. A topside contact on the termination trench allows a metal connector to be used to connect the termination trench to one of the polysilicon filled trenches in the active region so that the termination trench is biased (e.g., grounded). Additional doping under the termination trench is known to further reduce termination trench leakage.
Other MOSFET designs instead of using the polysilicon filled trenches as gate trenches, use the polysilicon filled trenches as field plates and provide a substantially planar FET structure, where the trench polysilicon is connected to source in the case of NMOS, not connected to the gate electrode as in vertical trenchFET. For example, these MOSFETs have a gate structure and a vertical drain drift region between polysilicon filled trenches configured to act as field plates sometimes referred to as “RESURF trenches”. For purposes of this patent application, the term “RESURF” is understood to refer to a material which reduces an electric field in an adjacent semiconductor region. A RESURF region may be for example a semiconductor region with an opposite conductivity type from the adjacent semiconductor region. RESURF structures are described in Appels, et.al., “Thin Layer High Voltage Devices” Philips J, Res. 35 1-13, 1980.
The RESURF trenches contain a dielectric liner and are generally filled with doped polysilicon. In the active region for n-channel MOSFETs the RESURF trenches (hereafter “active area trenches”) are polysilicon field plates which are electrically coupled to a source electrode. In the case of a n-channel MOSFET, there is a p-body region within an n-drift region on a substrate, where n-type dopants are in the source regions formed in the p-body region. The drain for the MOSFET can be a vertical drain drift region that uses the entire n-drift region below the p-body region, that has a drain contact on the bottom of the substrate, which can be an n+ substrate.
A contact metal stack makes electrical contact with a source region at lateral sides of the contact structure, makes electrical contact with a p-body contact region at a bottom surface of contact structure, and makes electrical contact to the polysilicon field plates in the active area trenches at the bottom surface of the contact structure. Another RESURF trench which is referred to as a “termination trench” is at a perimeter around the active area trenches.